Planimeter



J. A. WEISHAMPEL 1,994,843

PLANIMETER Filed June 25, 1955 INVENTOR Patented Mar. 19, 1935 UNITED STATES PATENT OFFICE 7 Claims.

This invention relates to area-integrating instruments and particularly to devices known as planimeters.

These instruments are extensively used on engine indicator-cards for determining mean effective pressures, and for measuring the area enclosed in irregularly-shaped figures.

Among the desirable features of a good planinieter are lightness, compactness, a minimum number of parts, and simplicity of construction to minimize cost.

In the present invention, all of these features have been incorporated without sacrificing precisi-on. Prior tothis invention, plan'imeters were otmore complicated construction, and were finished with such an exactness that their high cost placed them outside of the reach of most engineering students and mathematicians.

The primary object of the present invention, therefore, is to provide a planimeter of simplified construction to permit economical fabrication.

A second object, is to provide a light and compact structure through the proper selection of materials and careful design to allow for extreme portability.

Further objects and advantages will become apparent from the following description in conjunction with the appended drawing.

It is to be understood that the present disclosure is merely illustrative of a preferred form of the invention and it is not to be construed in a limiting sense, as I contemplate any structure properly within the scope of the appended claims.

Referring now to the drawing.

Fig. l is an isometric view showing a preferred form of the invention in operating position.

Fig. 2 is a plan View of the. apparatus in partially folded position.

Fig. 3 is an enlarged cross-sectional view taken on the lines 3--3 of Fig. 1.

Referring again to the drawing and more particularly to Figs. 1 and 2, the invention comprises a tracing arm 1, preferably made of celluloid or other light and transparent material. The tracing arm 1, is pivotally mounted in bracket 2, and is positively secured in a predetermined position by means of locking pin 3. The'locking pin 3 is shown threaded at its upper portion to engage the bracket, while its lower extremity depends into a circular recess 4 in the tracing arm to lock the arm in operating position. A spindle 5, comprising a rod of circular cross-section is secured at one end to the bracket 2. This spindle is provided with a smooth finish and is calibrated with uniformly spaced divisions. A close fitting sleeve 6 preferably made of hard transparent material such as glass, is mounted on the spindle and is adapted to revolve and to slide axially with respect to the spindle. The sleeve 6 is smooth on 5 the inside and is as close fitting to the spindle as is commensurate with a low degree of friction. Aset of annular marks on the inside of the sleeve serve as a Vernier scale or an index. The close fitting arrangement provides for proper co-axial 10 alignment of the sleeve and spindle, and minimizes error in reading produced by parallax.

A rotary member '7, shown as a double frustoconical wheel, is secured to the sleeve and is coaxial with it. This wheel is preferably of hard 15 material and should have a sharp periphery to engage with the surface which it rolls upon and to provide for axial motion with a minimum amount of slippage. The diiferential axial movement of sleeve and wheel with respect tothe 20 spindle is the only function of the area considered. Since the pure rotary motion is neglected, the di ameter of the wheel is inconsequential and is arbitrarily fixed Within limits which are compatible with neat design and proper function. Machin- 25 ing costs are thus minimized by the elimination of narrow limits of tolerance.

A cushion 8, of rubber or other resilient material is provided at either end of the spindle to absorb shock and to lessen the danger of damage 30 to the frangible sleeve.

A stop 9, at the outer extremity of the spindle serves as a means to keep the sleeve on the spindle, and also serves as a weight to maintain a pressure on the wheel.

The bracket 2 is flanged or rabbeted as shown in Fig. 3, .so that when the tracing arm is rotated to the operating position shown in Fig. 1, the arm is stopped, and the locking pin 3 becomes aligned with the circular recess 4, to maintain the tracing 40 arm at right angles to the spindle.

On the under surface of the transparent tracing arm 1, and at one or more distances from the bracket, a mark, or tracing point 10, is provided 45 to serve as a guide when tracing an outline. A stud or button 11 at the extremityv of the tracing arm serves as a means for the operator to grasp the instrument when in use.

A pivot link 12 is pivotally mounted in the 50 bracket 2, by means of a pivot screw 13 which also serves as fulcrum point for the tracing arm when pivoted from the folded to operating position. A thumb wheel 14 is shown on the present form to assist in holding the parts in proper operating -12 serves as a fixed fulcrum for that member.

The link is notched as at 17 to permit maximum swing without interfering with the locking pin 3.

As indicated in Fig. 2, the instrument may be folded up when not in use. The locking pin 3 is first unthreaded until the lower end is clear of the tracing arm, which is then free to be pivoted clockwise to the folded position. The tracing arm is grooved as at 18 to receive the wheel when the planimeter is folded and inhibits sliding motion of the sleeve.

In operation, the'tracing arm is secured in the operating position, perpendicular to the spindle, and the pivot link is secured to the operating surface at any convenient angle by means of pivot point 16. One or another tracing point 10 is next located at some predetermined and marked starting position on the figure to be measured, and the wheel manually moved axially to approximately a mid-position on the spindle. It has been found that the tracing point nearer the bracket gives slightly more accurate readings for small outlines thandoes the larger radius, but the latter is better adapted for larger figures. The figure is completely circumscribed until the tracing point again rests at the starting position. The area of the figure is determined as the product of a constant and the difference in readings.

The wheel worksbest on a quasi-polished surface, as smooth paper, finished wood, etc. As the precision of the instrumentis dependent upon the sensitiveness with which the sleeve slides upon the rod, greatv care has been taken to minimize weight and to maintain compactness of the moving parts. Parallax is avoided by taking readings directly from the spindle and without the use of cumbersome indicating devices, which detract from the sensitivity and introduce error. If desired the transparent sleeve may be made longer and provided with uniform markings, while the spindle is provided with a single index, or Vernier scale.

- The tracing point 10 on the underside of the tracing arm is spaced from the stud 11. This feature permits the figure to be traced without the hand of the operator causing interference, either mechanically or by obstructing the view. As the arm is preferably transparent, it is possible to Visualize the path which the tracing point is to follow for a short distance from any given position. 1

The tracing arm has been shown as transparent and the tracing point has been described as being underneath the arm. If desired, the tracing arm may be made of metal and the tracing point located at the terminus, or a hole may be drilled through the opaque arm to serve as a guide. If the transparent tracing arm is retained, the reference mark may be located on top of the arm, but slight error may be thus introduced. Similarly, the marks on the transparent sleeve may be made on the outer surface.

The spindle shown at right angles to the tracing arm in the preferred embodiment, may be positioned at other angles if desired. Compensation must be made in this case by considering the sine of the angle in the planimeter constant.

I claim:

1. A planimeter, comprising a tracing arm, a pivot link, a graduated spindle anglarly disposed to said tracing arm, a measuring wheel, on said spindle, and a transparent sleeve, rigidly secured to and co-axial with said measuring wheel, said transparent sleeve being provided with Vernier graduations on its inner periphery.

2. A planimeter, comprising a tracing arm, a pivot link, a cylindrical spindle disposed at right angles to said tracing arm, a measuring wheel, and a. transparent sleeve, rotatably and slidably mounted on said cylindrical spindle, being coaxial with saidmeasuring wheel and with said cylindricalspindle, said transparent sleeve being provided with a Vernier scale around its inner periphery, said spindle being provided on its outer periphery with uniformly spaced graduations.

3. A planimeter, comprising a tracing arm, a pivot link, a spindle angularly disposed to said tracing arm, a rotatable measuring wheel, a frangiblesleeve secured to said wheel, axially slidable on said cylindrical spindle, and a cushion of resilient material secured at either end of said'cylindrical spindle, and adapted to gently arrest the axial motion of said frangible sleeve.

4. A planimeter, comprising a bracket, a tracing, arm, a pivot link, said tracing arm and said pivot link being pivotally mounted to said bracket, a cylindrical spindlerigidly secured to said bracket, a measuring wheel freely mounted coaxially on said cylindrical spindle, and positive locking means to position said tracing arm perpendicular to said cylindrical spindle, said trac ing arm, said pivot link, and said cylindrical spindle, being adapted when in the unlocked position to fold one upon the other, said tracing arm being provided with a groove adapted to receive the outer periphery of said wheel when the planimeter is in thefolded position, to restrain said wheel from axial movement.

. 5. In an apparatus for integrating an' area laid out on a plane surface, having a rotatable wheel, and a pivot link, a fiat tracing arm to lie against said surface, said tracing arm being made of transparent material and being provided with a tracing point intermediate the ends ofsaid tracing arm.

6. In an apparatus for integrating an area laid out on a plane surface, having a rotatable wheel, and a pivot link, a flat tracing arm to lie against said surface, said tracing arm being made of transparent material and being provided with a tracing point on its lower surface, said tracing point being visible through said arm.

7. Ina planimeter having a pivot link, a spindle, and a rotatable measuring wheel on said spindle; a transparent tracing arm, angularly disposed'with respect to said spindle, and a tracing point on the bottom of said transparent tracing arm, said tracing point being visible thru said transparent tracing arm.

- JOHN A. WEISHAMPEL. 

